I've got a fairly recent model Time Capsule running backup for my Macbook Pro (Late 2013). Ever since upgrading to Yosemite I've noticed an occasional slow down when bringing up a file open or save dialog in most applications.
The delay is more than 5 seconds, and it only happens if a file dialog hasn't been displayed in a while. I can keep Finder windows open, and that doesn't seem to affect it.
The only pattern I've noticed is that I usually hear the Time Capsule drive spin up. Since I see the Time Capsule in the file dialog it seems like that is a likely candidate. It is rather annoying and would love to do something about it.
Time Capsule is only used for Time Machine purposes. I don't store or access any other files on it. It does not have any additional USB drives attached to it. My Macbook has a wired connection to the Time Capsule.